Pytest-cases

- Improved documentation to explain why `fixture` should be used instead of `pytest.fixture`. Fixed [125](https://github.com/smarie/python-pytest-cases/issues/125)

- Fixed ` ValueError: fixture is being applied more than once to the same function` when two functions parametrized with the same cases were sitting in the same file. Improved robustness when cases require fixtures, in particular when parametrized test/fixture sits in a class or when several of them sit in a class/module. Fixed [126](https://github.com/smarie/python-pytest-cases/issues/126)

- Added missing deprecation warning on `cases_generator`. Fixes [124](https://github.com/smarie/python-pytest-cases/issues/124).

- Removed `target` and `tags` arguments of `cases_generator` (deprecated api anyway) that were added by mistake in version 2.0.0 but never used.

- Added support for pytest items without funcargs. Fixes interoperability with other pytest plugins such as `pytest-black` or `pytest-flake8`. Fixes [122](https://github.com/smarie/python-pytest-cases/issues/122)

`pytest` 6 is now supported. Fixes [121](https://github.com/smarie/python-pytest-cases/issues/121)

Fixed issue with `parametrize_with_cases` when two cases with the same id and both requiring a fixture were to be created. Fixed [117](https://github.com/smarie/python-pytest-cases/issues/117).

Fixture closure engine refactoring:

- When no fixture unions are present, the fixture closure is now identical to the default one in `pytest`, to avoid issues originating from other plugins fiddling with the closure. Fixes [116](https://github.com/smarie/python-pytest-cases/issues/116)

- New `SuperClosure` class representing the "list" facade on top of the fixture tree (instead of `FixtureClosureNode`). In addition, this list facade now better handles editing the order of fixtures when possible. Fixes [111](https://github.com/smarie/python-pytest-cases/issues/111).

- Session and Module-scoped fixtures that are not used in all union alternatives are not any more torn town/setup across union alternatives. Fixes [120](https://github.com/smarie/python-pytest-cases/issues/120)

- Fixed `TypeError` with iterable argvalue in standard parametrize. Fixed [115](https://github.com/smarie/python-pytest-cases/issues/115).